[FIX] Avoid CacheMiss errors

Avoid CacheMiss errors setting a value on message_content field.

base_search_mail_content/models/mail_thread.py

@@ -29,10 +29,14 @@ class MailThread(models.AbstractModel):
 
     message_content = fields.Text(
         help="Message content, to be used only in searches",
-        compute=lambda self: False,
+        compute="_compute_message_content",
         search="_search_message_content",
     )
 
+    def _compute_message_content(self):
+        # Always assign a value to avoid CacheMiss errors
+        self.message_content = False
+
     @api.model
     def fields_view_get(
         self, view_id=None, view_type="form", toolbar=False, submenu=False